arm: Dump only kernel region and avoid dumping in user mode

Dump of memory surrounding register values can leak
kernel information in case of a user level fault.
Also virt_addr_valid() confirms whether a given virtual
address linearly maps to a kernel physical address.
Thus it offers a more exhaustive test for valid kernel
space then a check for is_vmalloc().

Change-Id: Ie394ec26e5ff7fd7bed1e7308a55b0033eddfe7c
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>